Refactor emphasis mark code in InlineTextBox.
Refactor emphasis mark code in InlineTextBox.

The duplication here is confusing; this separates a new paintEmphasisMark
function which contains the common logic.

Review URL: https://codereview.chromium.org/354263003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@177239 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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